Hello and welcome to JustAnswer.
Unfortunately, as an at-will employee, you could be terminated at any time for any reason with or without prior notice.
This stems from the employment at-will doctrine, which is codified in California Labor Code Section 2922, which states:
"An employment, having no specified term, may be terminated at the will of either party on notice to the other. Employment for a specified term means an employment for a period greater than one month."
However, if you applied for and received FMLA leave, then your employer would be prevented from taking any retalitory action against you,
including, and up to, termination, and you could file a compalint with the EEOC if you were terminated.
You can file a claim using the instructions available online here:http://www.eeoc.gov/employees/charge.cfm